The present invention relates to a power conversion device; in particular, relates to an improvement thereof to limit an inrush current while reducing overall weight and volume of the device even under a high rated power.
FIG. 16 is a circuit diagram showing a structure of a conventional power conversion device (which will be hereinafter referred to as xe2x80x9cfirst prior artxe2x80x9d) having an inrush current limiting circuit. This power conversion device 451 comprises an inverter 5, a capacitor 12, a main power source 10, switches 11, 11a, and a resistor 11b. The power conversion device 451 is utilized by connecting a three-phase inductive load 50, such as a motor, to output terminals U, V and W of the power conversion device 451.
The main power source 10 supplies source input terminals P, N of the inverter 5 with a DC (i.e. direct current) main power source voltage VDC1. The capacitor 12 is connected to the source input terminals P, N, and thereby functions to hold a DC voltage supplied to the inverter 5 at a constant level by means of its capacitance CDC. The switch 11, structured as a relay, functions as a main switch that turns ON when power supply to the inductive load 5 is started and turns OFF when stopped.
The switch 11a and the resistor 11b are interposed between the main power source 10 and the capacitor 12 so as to limit an inrush current which flows to charge the capacitor 12 immediately after the switch 11 turns on. After the switch 11 turns ON to start the inverter 5, the switch 11a remains OFF until charging of the capacitor 12 is almost completed. Accordingly, the inrush current is limited by the resistor 11b. 
When the charged voltage across the capacitor 12 almost reaches the main power source voltage VDC1, the switch 11a turns on. Thereafter, the inverter 5 starts its normal operation. Thus, a DC current is supplied to the inverter 5 by the main power source 10 with little loss, during the normal operation of the inverter 5.
However, since a large current supplied by the main power source 10 flows through the switch 11a and the resistor 11b, the switch 11a and the resistor 11b are required to have a high rated power, as well as the switch 11 as the main switch. In the power conversion device 451, therefore, it has been a problem that overall weight and volume of the device are large and a manufacturing cost thereof is high.
Further, since the switch 11 and the switch 11a (in particular, the switch 11) have to operate mechanically while a high voltage being applied, the reliability and durability thereof have been problematic. In particular, arcing is influential to the reliability and durability. Moreover, conduction losses of both the switch 11 and the switch 11a during the normal operation of the inverter 5 have also been not negligible concerns.
FIG. 17 is a circuit diagram showing a structure of another conventional power conversion device (which will be hereinafter referred to as xe2x80x9csecond prior artxe2x80x9d) having an inrush current limiting circuit. This power conversion device 452 is characteristically different from the power conversion device 451 according to the first prior art in that an NTC (negative, coefficient thermistor) 11c is interposed between the main power source 10 and the capacitor 12 in place of the switch 11a and the resistor 11b. 
Usually when starting the inverter 5, since the NTC 11c is low in temperature, it has high resistance. The inrush current flowing after the switch 11 turns ON is, therefore, limited by high resistance of the NTC 11c. The temperature of the NTC 11c rapidly rises up due to loss heat generated in the NTC 11c. Accordingly, the resistance of the NTC 11c rapidly decreases, which decreases loss in the NTC 11c. 
The loss generated in the NTC 11c during the operation of the inverter 5 is, however, not negligible, and therefore, the power conversion device 452 has been disadvantageously applicable only to devices having a low rated power (e.g. few kW or less). Further, if the inverter 5 starts operation before the NTC 11c has sufficiently been cooled down, e.g. the inverter 5 restarts immediately after it stopped, the NTC 11c does not sufficiently function as an inrush current limiter, which has also degraded the reliability of the device.
FIG. 18 is a circuit diagram showing a structure of still another conventional power conversion device (which will be hereinafter referred to as xe2x80x9cthird prior artxe2x80x9d) having an inrush current limiting circuit. This power conversion device 453 is disclosed in Japanese Patent Application Laid-Open No. 6-115836 (1994), and is characteristically different from any one of the power conversion devices 451 and 452 in that an initial charging circuit for charging the capacitor 12 is connected to the capacitor 12 in parallel.
The initial charging circuit comprises a reactor 11d, a switching element 11e, a DC power source 11f, a controller unit 11g, a resistor 11h, a base drive circuit 11j and a diode 11k. When the inverter 5 is started, the switch 11 is initially set OFF. During this period, the switching element 11e repeatedly turns ON and OFF due to the function of the controller unit 11g. As a result, a current is repeatedly charged into and discharged from the reactor 11d, and the discharged current is repeatedly supplied to the capacitor 12. Thus, the initial charging circuit functions as such a charge-pumping circuit as to charge up the capacitor 12. After the capacitor 12 almost completes charging, the initial charging circuit stops its operation and the switch 11 turns on. Thereafter, the inverter 5 starts the normal operation.
The power conversion device 453 is advantageously applicable to devices having a high rated power, and does advantageously not require any of the switch 11a and the resistor 11b for limiting the inrush current (see FIG. 16). However, the power conversion device 453 needs the reactor 11d and the switching element 11e, which is a power element, and therefore, the power conversion device 453 has been posing a problem in that overall weight and volume are large and a manufacturing cost is high, similarly to the power conversion device 451.
Accordingly, it is an object of the present invention to obtain a power conversion device which eliminates the above mentioned problems and limits an inrush current while reducing overall weight and volume of the device even under a high rated power.
In order to achieve the object, a first aspect of the present invention is directed to a power conversion device. The power conversion device comprises: a first switching element, one main electrode thereof being connected to a first source line; a first freewheeling diode connected to the first switching element in inverse-parallel; a second switching element, one main electrode thereof being connected to other main electrode of the first switching element, and other main electrode thereof being connected to a second source line; a second freewheeling diode connected to the second switching element in inverse-parallel; a third switching element, one main electrode thereof being connected to the first source line; a third freewheeling diode connected to the third switching element in inverse-parallel; a fourth switching element, one main electrode thereof being connected to other main electrode of the third switching element, and other main electrode thereof being connected to the second source line; a fourth freewheeling diode connected to the fourth switching element in inverse-parallel; a capacitor, one end and other end thereof being connected to the first source line and the second source line, respectively; a first switch, one end thereof being connected to one source line of a set of the first source line and the second source line; an initial charging circuit having a DC power source and a second switch connected in series, one end thereof being connected to a connection between the third switching element and the fourth switching element, and other end thereof being connected to the second source line; and an initial charge controller unit controlling the second switch and the second switching element so as to set the second switch ON and repeatedly turn ON and OFF the second switching element while the first switch is set OFF.
According to a second aspect of the present invention, in the power conversion device of the first aspect, the initial charge controller unit controls the second switching element to turn OFF when a charging current which is a current flowing through the initial charging circuit exceeds a first reference current and turn ON when the charging current drops below a second reference current.
According to a third aspect of the present invention, in the power conversion device of the second aspect, the initial charge controller unit controls the first switch and the second switch so as to set the first switch OFF and set the second switch ON when a charged voltage which is a voltage across the capacitor is lower than a reference voltage, and set the first switch ON and the second switch OFF when the charged voltage is higher than the reference voltage.
According to a fourth aspect of the present invention, in the power conversion device of the first aspect, the initial charging circuit further has a diode connected to the DC power source and the second switch in series.
According to a fifth aspect of the present invention, in the power conversion device of the first aspect, the power conversion device further comprises: a rectifier circuit converting an AC voltage input from an exterior into a DC voltage and applying the DC voltage between other source line of the set of the first source line and the second source line and other end of the first switch.
According to a sixth aspect of the present invention, in the power conversion device of the fifth aspect, the DC power source is a converter converting the DC voltage into another DC voltage.
According to a seventh aspect of the present invention, in the power conversion device of the first aspect, the power conversion device further comprises: first to fourth drive circuits respectively connected to control electrodes of the first to fourth switching elements, and respectively driving the first to fourth switching elements in response to first to fourth control signals respectively, wherein the initial charge controller unit -transmits the second control signal to the second drive circuit to thereby control the second switching element.
According to an eighth aspect of the present invention, in the power conversion device of the seventh aspect, source voltages of the second and fourth drive circuits are supplied by the DC power source.
According to a ninth aspect of the present invention, in the power conversion device of the third aspect, the power conversion device further comprises: a charged voltage detection circuit detecting the charged voltage to thereby output a first detection voltage, wherein the initial charge controller unit receives the first detection voltage to thereby control the first switch and the second switch on a basis of the charged voltage.
According to a tenth aspect of the present invention, in the power conversion device of the ninth aspect, the charged voltage detection circuit comprises: a first resistor, one end thereof being connected to the first source line; and a second resistor, one end thereof being connected to other end of the first resistor and other end thereof being connected to the second source line, and outputs a voltage at a connection between the first resistor and the second resistor as the first detection voltage.
According to an eleventh aspect of the present invention, in the power conversion device of the second aspect, the power conversion device further comprises: a charging current detection circuit detecting the charging current to thereby output a second detection voltage, wherein the initial charge controller unit receives the second detection voltage to thereby control the second switching element on a basis of the charging current.
According to a twelfth aspect of the present invention, in the power conversion device of the eleventh aspect, the charging current detection circuit comprises a third resistor interposed into a path of the charging current, and outputs a voltage drop across the third resistor as the second detection voltage.
According to a thirteenth aspect of the present invention, in the power conversion device of the third aspect, the power conversion device further comprises: a charged voltage detection circuit detecting the charged voltage to thereby output a first detection voltage; and a charging current detection circuit detecting the charging current to thereby output a second detection voltage, wherein the initial charge controller unit comprises: an A/D converter converting the first detection voltage and the second detection voltage from analogue form to digital form; a processing unit executing a digital operation processing on a basis of the first and second detection voltages having digital form to thereby calculate a set of signals which control the first switch, the second switch and the second switching element; and a buffer circuit amplifying the set of signals to thereby transmit the same to the first switch, the second switch and the second switching element.
According to a fourteenth aspect of the present invention, in the power conversion device of the third aspect, the processing unit comprises: a CPU operating on a basis of a program; and a memory storing the program, wherein the CPU operates on a basis of the program so as to calculate the set of signals.
According to a fifteenth aspect of the present invention, in the power conversion device of the third aspect, the power conversion device further comprises: a charged voltage detection circuit detecting the charged voltage to thereby output a first detection voltage; and a charging current detection circuit detecting the charging current to thereby output a second detection voltage, wherein the initial charge controller unit comprises: a first operational amplifier of two-input type; and a second operational amplifier of two-input type, wherein the first operational amplifier receives, at two inputs thereof, the first detection voltage and a first reference voltage, and transmits an output signal thereof to the first switch and the second switch so as to set only one of the first switch and the second switch ON, and the second operational amplifier has a positive feedback loop, receives the second detection voltage at a reverse input thereof, receives a second reference voltage at a non-reverse input thereof, and transmits an output signal thereof to the second switching element.
According to a sixteenth aspect of the present invention, in the power conversion device of the fifteenth aspect, the initial charge controller unit further comprises a logic switch, and the logic switch is interposed into a transmission path transmitting the output signal of the second operational amplifier to second switching element, and transmits the output signal of the second operational amplifier to the second switching element only when an output signal of the first operational amplifier is such a value to set the first switch OFF.
According to a seventeenth aspect of the present invention, in the power conversion device of the first aspect, the initial charge controller unit comprises: a CPU operating on a basis of a program; and a memory storing the program, wherein the CPU operates on a basis of the program so as to control the first switch, the second switch and the second switching element.
According to a nineteenth aspect of the present invention, in the power conversion device of the eighteenth aspect, the initial charge controller unit and the main controller unit are integrated into an integrated controller unit which comprises a CPU operating on a basis of a program and a memory storing the program, wherein the CPU operates on a basis of the program so as to perform both a control of the initial charge controller unit and a control of the main controller unit.
According to a twentieth aspect of the present invention, in the power conversion device of the first aspect, the power conversion device further comprises: a fifth switching element, one main electrode thereof being connected to the first source line; a fifth freewheeling diode connected to the fifth switching element in inverse-parallel; a sixth switching element, one main electrode thereof being connected to other main electrode of the fifth switching element, and other main electrode thereof being connected to the second source line; and a sixth freewheeling diode connected to the sixth switching element in inverse-parallel.
In a device according to the first aspect of the present invention, when the device is used, a main power source is connected between the other source line of the set of the first and second source lines and the other end of the first switch, and an inductive load is connected between two connections. One is a connection between the first and second switching elements, and the other is a connection between the third and fourth switching elements. When the first switch is set OFF, i.e. before the normal operation is started, the second switch is set ON and the second switching element repeatedly turns ON and OFF due to operation of the initial charge controller unit. As a result, the initial charge controller unit, the second switching element and an inductance of the inductive load function as a charge-pumping circuit, so that a charging current is repeatedly charged into and discharged from the inductance of the inductive load and the discharged charging current repeatedly charges the capacitor, passing through the first freewheeling diode. If the first switch is turned ON for the normal operation after the capacitor has been sufficiently charged, an inrush current due to turning-ON of the first switch can be suppressed.
Further, since charge-pumping function is utilized, a power resistor and a power switch through which a large current supplied by the main power source flows are removed contrary to the first prior art, and a rated current can be set large contrary to the second prior art. Moreover, since the second switching element and the first freewheeling diode which are indispensable for the normal operation and the inductance of the inductive load are utilized for implementing the charge-pumping function, additional switching element and inductor are not required contrary to the third prior art. Thus, in the device of the first aspect of the present invention, high reliability is achieved even under large rated current, and the overall weight and volume of the device can be reduced.
In a device according to the second aspect of the present invention, the second switching element repeatedly turns ON and OFF so that the charging current flowing through the initial charging circuit remains within a certain range. Accordingly, the capacitor is charged efficiently in a short time.
In a device according to the third aspect of the present invention, the capacitor is charged with the first switch set OFF and the second switch set ON until the charged voltage across the capacitor reaches the reference voltage. When the charged voltage exceeds the reference voltage, the second switch turns OFF so as to isolate the initial charging circuit from the inductive load, and the first switch turns ON so that the first and second source lines are supplied with a DC voltage from the main power source. Thus, charging of the capacitor and subsequent move to the normal operation are performed on the basis of the charged voltage across the capacitor, and therefore, the inrush current is suppressed with high reliability.
In a device according to the fourth aspect of the present invention, since the charging circuit is provided with a diode, the second switch and the DC power source are protected from high voltage being applied.
In a device according to the fifth aspect of the present invention, since the rectifier circuit is provided as a main electrode, the device can be used only by connecting an available AC power source without preparing an external DC power source.
In a device according to the sixth aspect of the present invention, since the DC power source is a converter which utilizes the DC voltage generated by the rectifier circuit, the DC power source is configured simply.
In a device according to the seventh aspect of the present invention, since the drive circuits for driving the switching elements are provided, small signals are satisfactory as the control signals transmitted to easily drive the switching elements having large rated current. Further, the initial charge controller unit controlling the second switching element can be configured simply.
In a device according to the eighth aspect of the present invention, since the second and fourth drive circuits are supplied with source voltage by the DC power source, an additional power source for supplying the second and fourth drive circuits with the source voltages is not required. Accordingly, the overall weight and volume and the manufacturing cost of the device are further reduced,
In a device according to the ninth aspect of the present invention, the charged voltage detection circuit is provided, and the detection signal output therefrom is utilized in the initial charge controller unit. The initial charge controller unit can, therefore, process a voltage signal having a value suited for processing, so that the initial charge controller unit can be configured simply.
In a device according to the tenth aspect of the present invention, the charged voltage detection circuit is formed simply of a serial circuit of the first and second resistors, and divided voltage of the charged voltage is output as the first detection voltage. Thus, the first detection voltage proportional to the charged voltage is obtained by a simple configuration. Further, the first detection voltage can easily be adjusted to a value suited for the processing of the initial charge controller unit by adjusting the resistance ratio of the first and second resistors.
In a device according to the eleventh aspect of the present invention, the charging current detection circuit is provided, and the detection signal output therefrom is utilized in the initial charge controller unit. The initial charge controller unit can, therefore, process a voltage signal having a value suited for processing, so that the initial charge controller unit can be configured simply.
In a device according to the twelfth aspect of the present invention, the charging current detection circuit comprises the third resistor interposed into the path of the charging current, and the voltage drop across the third resistor is output as the second detection voltage. Thus, the charging current detection circuit is configured simply.
In a device according to the thirteenth aspect of the present invention, the initial charge controller unit converts the first and second detection voltages into digital signals, and executes the digital operation processing on the basis of the digital signals to thereby perform the control operation. Accordingly, highly precise control operation with little secular change is implemented.
In a device according to the fourteenth aspect of the present invention, the CPU operates on the basis of the program stored in the memory so as to perform the control operation of the initial charge controller unit. The reference voltage, reference current, processing speed and other conditions can, therefore, be adjusted with high accuracy, and it is easy to adjust or modify these conditions. Thus, further precise control operation is implemented, and flexible design change suitable for so-called multi-item small-quantity production is facilitated.
In a device according to the fifteenth aspect of the present invention, the first and second operational amplifiers process the first and second detection voltages so as to perform the control operation of the initial charge controller unit. The initial charge controller unit is, therefore, configured simply and lightly, and the manufacturing cost of the device is reduced.
In a device according to the sixteenth aspect of the present invention, since the initial charge controller unit comprises the logic switch, the output signal of the second operational amplifier stops being stransmitted to the second switching element when the charged voltage has exceeded the reference voltage and the normal operation has been started. The initial charge controller unit, therefore, is prevented from intervening the normal operation of the first to fourth switching elements.
In a device according to the seventeenth aspect of the present invention, the CPU operates on the basis of the program stored in the memory so as to perform the control operation of the initial charge controller unit. The reference voltage, reference current, processing speed and other conditions can, therefore, be adjusted with high accuracy, and it is easy to adjust or modify these conditions. Thus, further precise control operation is implemented, and flexible design change suitable for so-called multi-item small-quantity production is facilitated.
In a device according to the eighteenth aspect of the present invention, since the main controller unit controlling the normal operation of the first to fourth switching elements is provided, it is not required to input signals driving the first to fourth switching elements from the exterior to perform the normal operation.
In a device according to the nineteenth aspect of the present invention, since the CPU operates on the basis of the program stored in the memory so as to perform the control operation of both the initial charge controller unit and the main controller unit. The main controller unit and the initial charge controller unit are configured simply and lightly as a whole, and the manufacturing cost of the device is reduced.
In a device according to the twentieth aspect of the present invention, since the fifth and sixth switching elements and the fifth and sixth freewheeling diodes are provided, a three-phase inductive load can be connected as the inductive load.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.